When a superconducting specimen makes the transition from the superconducting state to the normal (resistive) state, ohmic heating begins to occur. This ohmic heating causes the temperature of the specimen and its immediate surroundings to rise. This small temperature rise influences the specimen to move farther into the normal state causing increased ohmic heating. Thus, the shape of the voltage versus current (V-I) curve of a superconductor is influenced by its thermal surroundings. See, for example, Martinelli et al., "Investigation of Cryogenic Stability and Reliability of Operation of Nb.sub.3 Sn Coils in Helium Gas Environment" in Proceedings of the 1972 Applied Superconductivity Conference, Annapolis, Md., IEEE Publication No. 72CH0682-5-TABSC, pages 331-40 (1972), the entire disclosure of which is herein incorporated by reference.
The steepness of the voltage rise beyond the onset of flux flow is often represented by the power law relationship: EQU V=V.sub.0 (I/I.sub.0).sup.n (I)
wherein V.sub.0 and I.sub.0 are the voltage and current, respectively, just prior to the transition from the superconducting to the normal state. V is the voltage when measurement is taken. I is the corresponding current. The value of n can be determined from this relationship. Alternatively, n can be expressed by the following: EQU n=(I.multidot.d.sup.2 V/dI.sup.2)/(dV/dI)+1 (3)
For many superconductors, including high T.sub.c superconductors, n can be over 50. Clearly, n is strongly influenced by the thermal properties of the superconductor and the surrounding matrix material. In the extreme case of poor stabilization, however, the problem of thermal runaway can occur. In such instances, n is very large and the superconductor eventually vaporizes. Thus, conventional superconductor designs tend to minimize the dependance of n on the thermal surroundings.
It is known in the art to use superconductors as thermal sensors near T.sub.c. See Hu et al., "Design analysis of high T.sub.c superconducting microbolometer", Appl. Phys. Letter, 55:2444 (1989), the entire disclosure of which is incorporated by reference. As the sensor absorbs heat, the resistance changes rapidly as the sample becomes completely normal. Alternatively, the sample can be biased near the critical state by adjusting the current flow at any temperature below T.sub.c (J=0). The problem in such methods is that for larger currents, thermal runaway can destroy the sensor as it goes normal. To avoid this problem, a feedback circuit controlling the current flowing through the sample can be used.